Flat spectral response gratings using high index materials

ABSTRACT

An example head-mounted display device includes a plurality of optical elements in optical communication. The optical elements are configured to project an image in a field of view of a user wearing the head-mounted display device. A first optical element is configured to receive light from a second optical element. The first optical element defines a grating at along a periphery of the first optical element. The grating includes a plurality of protrusions extending from a base portion of the first optical element. The protrusions include a first material having a first optical dispersion profile for visible wavelengths of light. The grating also includes a second material disposed between at least some of the plurality of protrusions along the base portion of the first optical element. The second material has a second optical dispersion profile for visible wavelengths of light.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/919,718, filed Jul. 2, 2020, which claims the benefit of U.S.Provisional Application No. 62/889,650, filed Aug. 21, 2019, which ishereby incorporated by reference in its entirety.

TECHNICAL FIELD

This disclosure relates to optical elements having grating structuresand methods for producing the same.

BACKGROUND

Optical imaging systems, such as wearable display systems (e.g.,wearable display headsets) can include one or more eyepieces thatpresent projected images to a user. Eyepieces can be constructed usingthin layers of one or more highly refractive materials. As examples,eyepieces can be constructed from one or more layers of highlyrefractive glass, silicon, metal, or polymer substrates.

In some cases, an eyepiece can be patterned (e.g., with one or morelight diffractive nanostructures) such that it projects an imageaccording to a particular focal depth. For an example, to a user viewinga patterned eyepiece, the projected image can appear to be a particulardistance away from the user.

Further, multiple eyepieces can be used in conjunction to project asimulated three-dimensional image. For example, multiple eyepieces—eachhaving a different pattern—can be layered one atop another, and eacheyepiece can project a different depth layer of a volumetric image.Thus, the eyepieces can collectively present the volumetric image to theuser across three-dimensions. This can be useful, for example, inpresenting the user with a “virtual reality” environment.

SUMMARY

This disclosure describes optical elements having certain gratingstructures and methods for producing the same. One or more of thedescribed implementations can be used to produce optical elements thatexhibit a substantially constant diffraction efficiency across aparticular spectrum (e.g., the visible spectrum). In someimplementations, the optical elements can be suitable for use aseyepieces in a wearable display headset.

In an example implementation, an optical element includes one or moregrating structures defined along its periphery (e.g., along an interfacebetween the optical element and another optical element, or along aninterface between the optical element and air). The grating structuresare formed from one or more high index materials, such as titaniumdioxide (TiO₂), silicon carbide (SiC), and/or lithium niobate (LiNbO₃).In particular, the differential dispersion of these materials can beused to achieve a uniform diffraction efficiency across the visiblespectrum. This can be beneficial, for example, in fabricating of singlewaveguide layer eyepieces (e.g., for use in a wearable display headset)that can display a high-quality multi-color image (e.g., ared-green-blue image) having high color uniformity over a wide field ofview.

In an aspect, a head-mounted display device includes a plurality ofoptical elements in optical communication. The plurality of opticalelements is configured, during operation of the head-mounted displaydevice, to project an image in a field of view of a user wearing thehead-mounted display device. A first optical element of the plurality ofoptical elements is configured to receive light from a second opticalelement of the plurality of optical elements. The first optical elementdefines a grating at along a periphery of the first optical element. Thegrating includes a plurality of protrusions extending from a baseportion of the first optical element. The protrusions include a firstmaterial having a first optical dispersion profile for visiblewavelengths of light. The grating also includes a second materialdisposed between at least some of the plurality of protrusions along thebase portion of the first optical element. The second material has asecond optical dispersion profile for visible wavelengths of light.

Implementations of this aspect can include one or more of the followingfeatures.

In some implementations, the second material can be titanium dioxide(TiO₂).

In some implementations, the first material can be silicon carbide(SiC).

In some implementations, the first material can be lithium niobate(LiNbO₃).

In some implementations, the base portion of the first optical elementcan include the first material.

In some implementations, the base portion of the first optical elementcan be composed of the same material as the plurality of protrusions.

In some implementations, the base portion of the first optical elementcan be integrally formed with the plurality of protrusions.

In some implementations, each protrusion can have a substantiallyrectangular cross-section.

In some implementations, each protrusion can extend a first height abovea surface of the base portion of the first optical element. The secondmaterial can extend a second height above the surface of the baseportion of the first optical element, the second height being differentfrom the first height.

In some implementations, the first height can be greater than the secondheight.

In some implementations, the first height can be approximately 90 nm.

In some implementations, the second height can be approximately 80 nm.

In some implementations, the grating can repeat according to a periodalong a length of the base portion of the first optical element.

In some implementations, the period can correspond to a length ofapproximately 208 nm.

In some implementations, each protrusion can have a substantially equalwidth.

In some implementations, each protrusion can have a width ofapproximately 140 nm.

In some implementations, the first and second optical dispersionprofiles can reduce variations between a first diffraction efficiency ofthe grating with respect to a first wavelength of incident light, asecond diffraction efficiency of the grating with respect to a secondwavelength of incident light, and a third diffraction efficiency of thegrating with respect to a third wavelength of incident light withrespect to a range of incident angles compared to a grating composed ofonly the first material.

In some implementations, the first wavelength can correspond to firstcolor in the visible spectrum, the second wavelength can correspond to asecond color in the visible spectrum, and the third wavelength cancorresponds to third color in the visible spectrum, the first color, thesecond color, and third color being different from one another.

In some implementations, the first color can be red, the second colorcan be green, and the third color can be blue.

In some implementations, the range of incident angles can beapproximately −20° to 20°.

In another aspect, a method of constructing a head-mounted displaydevice includes providing a first optical element including a gratingformed along a first surface of the first optical element. The gratingincludes a plurality of protrusions including a first material having afirst optical dispersion profile for visible wavelengths of light, and asecond material deposited between at least some of the plurality ofprotrusions along the first surface of the first optical element. Thesecond material has a second optical dispersion profile for visiblewavelengths of light, The method also includes positioning the firstoptical element in optical communication with a second optical elementin the head-mounted display device.

Implementations of this aspect can include one or more of the followingfeatures.

In some implementations, the second material can be titanium dioxide(TiO₂).

In some implementations, the first material can be silicon carbide(SiC).

In some implementations, the first material can be lithium niobate(LiNbO₃).

In some implementations, the grating can be formed by etching aplurality of channels onto the first optical element along the firstsurface, each channel having a first depth, and depositing the secondmaterial between at least some of the plurality of protrusions along thefirst surface.

In some implementations, each channel can have a substantiallyrectangular cross-section.

In some implementations, each channel can have a substantially equalwidth.

In some implementations, each channel can have a width of approximately68 nm.

In some implementations depositing the second material can includedepositing the second material into at least some of the channels.

In some implementations, depositing the second material can includesputtering the second material into at least some of the channels.

In some implementations, the second material can be deposited such thatit extends a first height within the channel.

In some implementations, the first depth can be greater than the firstheight.

In some implementations, the first depth can be approximately 90 nm.

In some implementations, the first height can be approximately 80 nm.

In some implementations, the grating can be formed according to a periodalong a length of the first surface.

In some implementations, the period can correspond to a length ofapproximately 208 nm.

The details of one or more embodiments are set forth in the accompanyingdrawings and the description below. Other features and advantages willbe apparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 shows an example of a wearable display system.

FIG. 2A shows a conventional display system for simulatingthree-dimensional image data for a user.

FIG. 2B shows aspects of an approach for simulating three-dimensionalimage data using multiple depth planes.

FIGS. 3A-3C show relationships between radius of curvature and focalradius.

FIG. 4 shows an example of a waveguide stack for outputting imageinformation to a user in an AR eyepiece.

FIGS. 5 and 6 show examples of exit beams outputted by a waveguide.

FIG. 7 shows, in cross-section, an example grating structure.

FIG. 8A shows, in cross-section, an example repeating unit of a gratingstructure.

FIGS. 8B and 8C show, in cross-section, example optical elements havingthe repeating units shown in FIG. 5A.

FIGS. 9A and 9B show the angular response of example grating structures.

FIGS. 10A and 10B show intensity maps of light emitted by exampleeyepieces having example grating structures.

FIG. 11 shows the refractive indices of example materials used to formgrating structures described herein.

FIGS. 12A and 12B show example repeating units of a grating structure.

FIG. 13 is a flow chart diagrams of an example process for constructinga head-mounted display device using the optical elements and gratingstructures described herein

FIG. 14 is a diagram of an example computer system.

DETAILED DESCRIPTION

FIG. 1 illustrates an example wearable display system 60 thatincorporates a high index material grating. The display system 60includes a display or eyepiece 70, and various mechanical and electronicmodules and systems to support the functioning of that display 70. Thedisplay 70 may be coupled to a frame 80, which is wearable by a displaysystem user 90 and which is configured to position the display 70 infront of the eyes of the user 90. The display 70 may be consideredeyewear in some embodiments. In some embodiments, a speaker 100 iscoupled to the frame 80 and is positioned adjacent the ear canal of theuser 90. The display system may also include one or more microphones 110to detect sound. The microphone 110 can allow the user to provide inputsor commands to the system 60 (e.g., the selection of voice menucommands, natural language questions, etc.), and/or can allow audiocommunication with other persons (e.g., with other users of similardisplay systems). The microphone 110 can also collect audio data fromthe user's surroundings (e.g., sounds from the user and/or environment).In some embodiments, the display system may also include a peripheralsensor 120 a, which may be separate from the frame 80 and attached tothe body of the user 90 (e.g., on the head, torso, an extremity, etc.).The peripheral sensor 120 a may acquire data characterizing thephysiological state of the user 90 in some embodiments.

The display 70 is operatively coupled by a communications link 130, suchas by a wired lead or wireless connectivity, to a local data processingmodule 140 which may be mounted in a variety of configurations, such asfixedly attached to the frame 80, fixedly attached to a helmet or hatworn by the user, embedded in headphones, or removably attached to theuser 90 (e.g., in a backpack-style configuration or in a belt-couplingstyle configuration). Similarly, the sensor 120 a may be operativelycoupled by communications link 120 b (e.g., a wired lead or wirelessconnectivity) to the local processor and data module 140. The localprocessing and data module 140 may include a hardware processor, as wellas digital memory, such as non-volatile memory (e.g., flash memory or ahard disk drive), both of which may be utilized to assist in theprocessing, caching, and storage of data. The data may include data 1)captured from sensors (which may be, e.g., operatively coupled to theframe 80 or otherwise attached to the user 90), such as image capturedevices (e.g., cameras), microphones, inertial measurement units,accelerometers, compasses, GPS units, radio devices, gyros, and/or othersensors disclosed herein; and/or 2) acquired and/or processed using aremote processing module 150 and/or a remote data repository 160(including data relating to virtual content), possibly for passage tothe display 70 after such processing or retrieval. The local processingand data module 140 may be operatively coupled by communication links170, 180, such as via a wired or wireless communication links, to theremote processing module 150 and the remote data repository 160 suchthat these remote modules 150, 160 are operatively coupled to each otherand available as resources to the local processing and data module 140.In some embodiments, the local processing and data module 140 mayinclude one or more of the image capture devices, microphones, inertialmeasurement units, accelerometers, compasses, GPS units, radio devices,and/or gyros. In some other embodiments, one or more of these sensorsmay be attached to the frame 80, or may be standalone devices thatcommunicate with the local processing and data module 140 by wired orwireless communication pathways.

The remote processing module 150 may include one or more processors toanalyze and process data, such as image and audio information. In someembodiments, the remote data repository 160 may be a digital datastorage facility, which may be available through the internet or othernetworking configuration in a “cloud” resource configuration. In someembodiments, the remote data repository 160 may include one or moreremote servers, which provide information (e.g., information forgenerating augmented reality content) to the local processing and datamodule 140 and/or the remote processing module 150. In otherembodiments, all data is stored and all computations are performed inthe local processing and data module, allowing fully autonomous use froma remote module.

The perception of an image as being “three-dimensional” or “3-D” may beachieved by providing slightly different presentations of the image toeach eye of the user. FIG. 2A illustrates a conventional display systemfor simulating three-dimensional image data for a user. Two distinctimages 190, 200—one for each eye 210, 220—are output to the user. Theimages 190, 200 are spaced from the eyes 210, 220 by a distance 230along an optical or z-axis that is parallel to the line of sight of theuser. The images 190, 200 are flat and the eyes 210, 220 may focus onthe images by assuming a single accommodated state. Such 3-D displaysystems rely on the human visual system to combine the images 190, 200to provide a perception of depth and/or scale for the combined image.

However, the human visual system is complicated and providing arealistic perception of depth is challenging. For example, many users ofconventional “3-D” display systems find such systems to be uncomfortableor may not perceive a sense of depth at all. Objects may be perceived asbeing “three-dimensional” due to a combination of vergence andaccommodation. Vergence movements (e.g., rotation of the eyes so thatthe pupils move toward or away from each other to converge therespective lines of sight of the eyes to fixate upon an object) of thetwo eyes relative to each other are closely associated with focusing (or“accommodation”) of the lenses of the eyes. Under normal conditions,changing the focus of the lenses of the eyes, or accommodating the eyes,to change focus from one object to another object at a differentdistance will automatically cause a matching change in vergence to thesame distance, under a relationship known as the “accommodation-vergencereflex,” as well as pupil dilation or constriction. Likewise, undernormal conditions, a change in vergence will trigger a matching changein accommodation of lens shape and pupil size. As noted herein, manystereoscopic or “3-D” display systems display a scene using slightlydifferent presentations (and, so, slightly different images) to each eyesuch that a three-dimensional perspective is perceived by the humanvisual system. Such systems can be uncomfortable for some users,however, since they simply provide image information at a singleaccommodated state and work against the “accommodation-vergence reflex.”Display systems that provide a better match between accommodation andvergence may form more realistic and comfortable simulations ofthree-dimensional image data.

FIG. 2B illustrates aspects of an approach for simulatingthree-dimensional image data using multiple depth planes. With referenceto FIG. 2B, the eyes 210, 220 assume different accommodated states tofocus on objects at various distances on the z-axis. Consequently, aparticular accommodated state may be said to be associated with aparticular one of the illustrated depth planes 240, which has anassociated focal distance, such that objects or parts of objects in aparticular depth plane are in focus when the eye is in the accommodatedstate for that depth plane. In some embodiments, three-dimensional imagedata may be simulated by providing different presentations of an imagefor each of the eyes 210, 220, and also by providing differentpresentations of the image corresponding to multiple depth planes. Whilethe respective fields of view of the eyes 210, 220 are shown as beingseparate for clarity of illustration, they may overlap, for example, asdistance along the z-axis increases. In addition, while the depth planesare shown as being flat for ease of illustration, it will be appreciatedthat the contours of a depth plane may be curved in physical space, suchthat all features in a depth plane are in focus with the eye in aparticular accommodated state.

The distance between an object and an eye 210 or 220 may also change theamount of divergence of light from that object, as viewed by that eye.FIGS. 3A-3C illustrate relationships between distance and the divergenceof light rays. The distance between the object and the eye 210 isrepresented by, in order of decreasing distance, R1, R2, and R3. Asshown in FIGS. 3A-3C, the light rays become more divergent as distanceto the object decreases. As distance increases, the light rays becomemore collimated. Stated another way, it may be said that the light fieldproduced by a point (the object or a part of the object) has a sphericalwavefront curvature, which is a function of how far away the point isfrom the eye of the user. The curvature increases with decreasingdistance between the object and the eye 210. Consequently, at differentdepth planes, the degree of divergence of light rays is also different,with the degree of divergence increasing with decreasing distancebetween depth planes and the user's eye 210. While only a single eye 210is illustrated for clarity of illustration in FIGS. 3A-3C and otherfigures herein, it will be appreciated that the discussions regardingthe eye 210 may be applied to both eyes 210 and 220 of a user.

A highly believable simulation of perceived depth may be achieved byproviding, to the eye, different presentations of an image correspondingto each of a limited number of depth planes. The different presentationsmay be separately focused by the user's eye, thereby helping to providethe user with depth cues based on the amount of accommodation of the eyerequired to bring into focus different image features for the scenelocated on different depth planes and/or based on observing differentimage features on different depth planes being out of focus.

FIG. 4 illustrates an example of a waveguide stack for outputting imageinformation to a user in an AR eyepiece. A display system 250 includes astack of waveguides, or stacked waveguide assembly, 260 that may beutilized to provide three-dimensional perception to the eye/brain usinga plurality of waveguides 270, 280, 290, 300, 310. In some embodiments,the display system 250 is the system 60 of FIG. 1, with FIG. 4schematically showing some parts of that system 60 in greater detail.For example, the waveguide assembly 260 may be part of the display 70 ofFIG. 1. It will be appreciated that the display system 250 may beconsidered a light field display in some embodiments.

The waveguide assembly 260 may also include a plurality of features 320,330, 340, 350 between the waveguides. In some embodiments, the features320, 330, 340, 350 may be one or more lenses. The waveguides 270, 280,290, 300, 310 and/or the plurality of lenses 320, 330, 340, 350 may beconfigured to send image information to the eye with various levels ofwavefront curvature or light ray divergence. Each waveguide level may beassociated with a particular depth plane and may be configured to outputimage information corresponding to that depth plane. Image injectiondevices 360, 370, 380, 390, 400 may function as a source of light forthe waveguides and may be utilized to inject image information into thewaveguides 270, 280, 290, 300, 310, each of which may be configured, asdescribed herein, to distribute incoming light across each respectivewaveguide, for output toward the eye 210. Light exits an output surface410, 420, 430, 440, 450 of each respective image injection device 360,370, 380, 390, 400 and is injected into a corresponding input surface460, 470, 480, 490, 500 of the respective waveguides 270, 280, 290, 300,310. In some embodiments, the each of the input surf aces 460, 4 70,480, 490, 500 may be an edge of a corresponding waveguide, or may bepart of a major surface of the corresponding waveguide (that is, one ofthe waveguide surfaces directly facing the world 510 or the user's eye210). In some embodiments, a beam of light (e.g., a collimated beam) maybe injected into each waveguide and may be replicated, such as bysampling into beamlets by diffraction, in the waveguide and thendirected toward the eye 210 with an amount of optical powercorresponding to the depth plane associated with that particularwaveguide. In some embodiments, a single one of the image injectiondevices 360, 370, 380, 390, 400 may be associated with, and inject lightinto, a plurality (e.g., three) of the waveguides 270, 280, 290, 300,310.

In some embodiments, the image injection devices 360, 370, 380, 390, 400are discrete displays that each produce image information for injectioninto a corresponding waveguide 270, 280, 290, 300, 310, respectively. Insome other embodiments, the image injection devices 360, 370, 380, 390,400 are the output ends of a single multiplexed display which maytransmit image information via one or more optical conduits (such asfiber optic cables) to each of the image injection devices 360, 370,380, 390, 400. It will be appreciated that the image informationprovided by the image injection devices 360, 370, 380, 390, 400 mayinclude light of different wavelengths, or colors.

In some embodiments, the light injected into the waveguides 270, 280,290, 300, 310 is provided by a light projector system 520, whichincludes a light module 530, which may include a light source or lightemitter, such as a light emitting diode (LED). The light from the lightmodule 530 may be directed to, and modulated by, a light modulator 540(e.g., a spatial light modulator), via a beamsplitter (BS) 550. Thelight modulator 540 may spatially and/or temporally change the perceivedintensity of the light injected into the waveguides 270, 280, 290, 300,310. Examples of spatial light modulators include liquid crystaldisplays (LCD), including a liquid crystal on silicon (LCOS) displays,and digital light processing (DLP) displays.

In some embodiments, the light projector system 520, or one or morecomponents thereof, may be attached to the frame 80 (FIG. 1). Forexample, the light projector system 520 may be part of a temporalportion (e.g., ear stem 82) of the frame 80 or disposed at an edge ofthe display 70. In some embodiments, the light module 530 may beseparate from the BS 550 and/or light modulator 540.

In some embodiments, the display system 250 may be a scanning fiberdisplay comprising one or more scanning fibers to project light invarious patterns (e.g., raster scan, spiral scan, Lissajous patterns,etc.) into one or more waveguides 270, 280, 290, 300, 310 and ultimatelyinto the eye 210 of the user. In some embodiments, the illustrated imageinjection devices 360, 370, 380, 390, 400 may schematically represent asingle scanning fiber or a bundle of scanning fibers configured toinject light into one or a plurality of the waveguides 270, 280, 290,300, 310. In some other embodiments, the illustrated image injectiondevices 360, 370, 380, 390, 400 may schematically represent a pluralityof scanning fibers or a plurality of bundles of scanning fibers, each ofwhich are configured to inject light into an associated one of thewaveguides 270, 280, 290, 300, 310. One or more optical fibers maytransmit light from the light module 530 to the one or more waveguides270, 280, 290, 300, and 310. In addition, one or more interveningoptical structures may be provided between the scanning fiber, orfibers, and the one or more waveguides 270, 280, 290, 300, 310 to, forexample, redirect light exiting the scanning fiber into the one or morewaveguides 270,280,290,300,310.

A controller 560 controls the operation of the stacked waveguideassembly 260, including operation of the image injection devices 360,370, 380, 390, 400, the light source 530, and the light modulator 540.In some embodiments, the controller 560 is part of the local dataprocessing module 140. The controller 560 includes programing (e.g.,instructions in a non-transitory medium) that regulates the timing andprovision of image information to the waveguides 270, 280, 290, 300,310. In some embodiments, the controller may be a single integraldevice, or a distributed system connected by wired or wirelesscommunication channels. The controller 560 may be part of the processingmodules 140 or 150 (FIG. 1) in some embodiments.

The waveguides 270, 280, 290, 300, 310 may be configured to propagatelight within each respective waveguide by total internal reflection(TIR). The waveguides 270, 280, 290, 300, 310 may each be planar or haveanother shape (e.g., curved), with major top and bottom surfaces andedges extending between those major top and bottom surfaces. In theillustrated configuration, the waveguides 270, 280, 290, 300, 310 mayeach include out-coupling optical elements 570, 580, 590, 600, 610 thatare configured to extract light out of a waveguide by redirecting thelight, propagating within each respective waveguide, out of thewaveguide to output image information to the eye 210. Extracted lightmay also be referred to as out-coupled light and the out-couplingoptical elements light may also be referred to light extracting opticalelements. An extracted beam of light may be output by the waveguide atlocations at which the light propagating in the waveguide strikes alight extracting optical element. The out-coupling optical elements 570,580, 590, 600, 610 may be, for example, diffractive optical features,including diffractive gratings, as discussed further herein. While theout-coupling optical elements 570, 580, 590, 600, 610 are illustrated asbeing disposed at the bottom major surfaces of the waveguides 270, 280,290, 300, 310, in some embodiments they may be disposed at the topand/or bottom major surfaces, and/or may be disposed directly in thevolume of the waveguides 270, 280, 290, 300, 310, as discussed furtherherein. In some embodiments, the out-coupling optical elements 570, 580,590, 600, 610 may be formed in a layer of material that is attached to atransparent substrate to form the waveguides 270, 280, 290, 300, 310. Insome other embodiments, the waveguides 270, 280, 290, 300, 310 may be amonolithic piece of material and the out-coupling optical elements 570,580, 590, 600, 610 may be formed on a surface and/or in the interior ofthat piece of material.

Each waveguide 270, 280, 290, 300, 310 may output light to form an imagecorresponding to a particular depth plane. For example, the waveguide270 nearest the eye may deliver collimated beams of light to the eye210. The collimated beams of light may be representative of the opticalinfinity focal plane. The next waveguide up 280 may output collimatedbeams of light which pass through the first lens 350 (e.g., a negativelens) before reaching the eye 210. The first lens 350 may add a slightconvex wavefront curvature to the collimated beams so that the eye/braininterprets light coming from that waveguide 280 as originating from afirst focal plane closer inward toward the eye 210 from opticalinfinity. Similarly, the third waveguide 290 passes its output lightthrough both the first lens 350 and the second lens 340 before reachingthe eye 210. The combined optical power of the first lens 350 and thesecond lens 340 may add another incremental amount of wavefrontcurvature so that the eye/brain interprets light coming from the thirdwaveguide 290 as originating from a second focal plane that is evencloser inward from optical infinity than was light from the secondwaveguide 280.

The other waveguide layers 300, 310 and lenses 330, 320 are similarlyconfigured, with the highest waveguide 310 in the stack sending itsoutput through all of the lenses between it and the eye for an aggregatefocal power representative of the closest focal plane to the person. Tocompensate for the stack of lenses 320, 330, 340, 350 whenviewing/interpreting light coming from the world 510 on the other sideof the stacked waveguide assembly 260, a compensating lens layer 620 maybe disposed at the top of the stack to compensate for the aggregateoptical power of the lens stack 320, 330, 340, 350 below. Such aconfiguration provides as many perceived focal planes as there areavailable waveguide/lens pairings. Both the out-coupling opticalelements of the waveguides and the focusing aspects of the lenses may bestatic (i.e., not dynamic or electro-active). In some alternativeembodiments, either or both may be dynamic using electro-activefeatures.

In some embodiments, two or more of the waveguides 270, 280, 290, 300,310 may have the same associated depth plane. For example, multiplewaveguides 270, 280, 290, 300, 310 may output images set to the samedepth plane, or multiple subsets of the waveguides 270, 280, 290, 300,310 may output images set to the same plurality of depth planes, withone set for each depth plane. This can provide advantages for forming atiled image to provide an expanded field of view at those depth planes.

The out-coupling optical elements 570, 580, 590, 600, 610 may beconfigured to both redirect light out of their respective waveguides andto output this light with the appropriate amount of divergence orcollimation for a particular depth plane associated with the waveguide.As a result, waveguides having different associated depth planes mayhave different configurations of out-coupling optical elements 570, 580,590, 600, 610, which output light with a different amount of divergencedepending on the associated depth plane. In some embodiments, the lightextracting optical elements 570, 580, 590, 600, 610 may be volumetric orsurface features, which may be configured to output light at specificangles. For example, the light extracting optical elements 570, 580,590, 600, 610 may be volume holograms, surface holograms, and/ordiffraction gratings. In some embodiments, the features 320, 330, 340,350 may not be lenses; rather, they may simply be spacers (e.g.,cladding layers and/or structures for forming air gaps).

In some embodiments, the out-coupling optical elements 570, 580, 590,600, 610 are diffractive features with a diffractive efficiencysufficiently low such that only a portion of the power of the light in abeam is re-directed toward the eye 210 with each interaction, while therest continues to move through a waveguide via TIR. Accordingly, theexit pupil of the light module 530 is replicated across the waveguide tocreate a plurality of output beams carrying the image information fromlight source 530, effectively expanding the number of locations wherethe eye 210 may intercept the replicated light source exit pupil. Thesediffractive features may also have a variable diffractive efficiencyacross their geometry to improve uniformity of light output by thewaveguide.

In some embodiments, one or more diffractive features may be switchablebetween “on” states in which they actively diffract, and “off” states inwhich they do not significantly diffract. For instance, a switchablediffractive element may include a layer of polymer dispersed liquidcrystal in which microdroplets form a diffraction pattern in a hostmedium, and the refractive index of the microdroplets may be switched tosubstantially match the refractive index of the host material (in whichcase the pattern does not appreciably diffract incident light) or themicrodroplet may be switched to an index that does not match that of thehost medium (in which case the pattern actively diffracts incidentlight).

In some embodiments, a camera assembly 630 (e.g., a digital camera,including visible light and IR light cameras) may be provided to captureimages of the eye 210, parts of the eye 210, or at least a portion ofthe tissue surrounding the eye 210 to, for example, detect user inputs,extract biometric information from the eye, estimate and track the gazedirection of the eye, to monitor the physiological state of the user,etc. In some embodiments, the camera assembly 630 may include an imagecapture device and a light source to project light (e.g., IR or near-IRlight) to the eye, which may then be reflected by the eye and detectedby the image capture device. In some embodiments, the light sourceincludes light emitting diodes (“LEDs”), emitting in IR or near-IR. Insome embodiments, the camera assembly 630 may be attached to the frame80 (FIG. 1) and may be in electrical communication with the processingmodules 140 or 150, which may process image information from the cameraassembly 630 to make various determinations regarding, for example, thephysiological state of the user, the gaze direction of the wearer, irisidentification, etc. In some embodiments, one camera assembly 630 may beutilized for each eye, to separately monitor each eye.

FIG. 5 illustrates an example of exit beams output by a waveguide. Onewaveguide is illustrated (with a perspective view), but other waveguidesin the waveguide assembly 260 (FIG. 4) may function similarly. Light 640is injected into the waveguide 270 at the input surface 460 of thewaveguide 270 and propagates within the waveguide 270 by TIR. Throughinteraction with diffractive features, light exits the waveguide as exitbeams 650. The exit beams 650 replicate the exit pupil from a projectordevice which projects images into the waveguide. Any one of the exitbeams 650 includes a sub-portion of the total energy of the input light640. And in a perfectly efficient system, the summation of the energy inall the exit beams 650 would equal the energy of the input light 640.The exit beams 650 are illustrated as being substantially parallel inFIG. 6 but, as discussed herein, some amount of optical power may beimparted depending on the depth plane associated with the waveguide 270.Parallel exit beams may be indicative of a waveguide with out-couplingoptical elements that out-couple light to form images that appear to beset on a depth plane at a large distance (e.g., optical infinity) fromthe eye 210. Other waveguides or other sets of out-coupling opticalelements may output an exit beam pattern that is more divergent, asshown in FIG. 6, which would require the eye 210 to accommodate to acloser distance to bring it into focus on the retina and would beinterpreted by the brain as light from a distance closer to the eye 210than optical infinity.

Additional information regarding wearable display systems (e.g.,including optical elements used in wearable display systems) can befound in U.S. patent application Ser. No. 16/221,359, filed Dec. 14,2018, and entitled “EYEPIECES FOR AUGMENTED REALITY DISPLAY SYSTEM,” thecontents of which are incorporated by reference in their entirety.

As noted above, wearable display system 60 includes one or more opticalelements having one or more grating structures that enhance an opticalperformance of the wearable display system. As an example, one or moreoptical elements forming an eyepiece of the wearable display system 60,such as the waveguide stack shown in FIG. 4, can include gratingsstructures defined along their peripheries (e.g., along an interfacebetween an optical element and another optical element, or along aninterface between an optical element and air, such as out-couplingoptical elements 570, 580, 590, 600, 610), and formed from one or morehigh index materials, such as such as titanium dioxide (TiO₂), siliconcarbide (SiC), and/or lithium niobate (LiNbO₃). In particular, thedifferential dispersion of these materials can be used to achieve auniform diffraction efficiency across the visible spectrum. This can bebeneficial, for example, in fabricating of single layer eyepieces (e.g.,for use in the wearable display system) that can display a high-qualitymulti-color image (e.g., a red-green-blue image) having high coloruniformity over a wide field of view. For instance, referring to FIG. 4,each of the waveguides 270, 280, 290, 300, 310 can be configured to sendimage information to the eye according to multiple wavelengths of light(e.g., corresponding to a red-green-blue image).

In general, high refractive index substrates (such as LiNbO₃ or SiC)offer the possibility of multiplexing the full red-green-blue (RGB)spectrum onto a single layer substrate. However, the use of certaintypes of grating structures, such as binary or blazed structures, canresult poor eyepiece performance, as such grating structures may impartcolor-selective properties onto the eyepiece. For instance, the gratingstructures may cause shorter wavelengths of light to be diffracted moreefficiently compared to longer wavelengths of light. This effect can beundesirable in some circumstances. For example, this effect may make itmore difficult to obtain a good color balance or uniformity in singlelayer RGB eyepiece while maintaining an acceptable eyepiece efficiency.

However, as described herein, the differential dispersion of certainhigh index materials can be used to achieve uniform diffractionefficiency across the visible spectrum. This enables the fabrication ofefficient single layer RGB eyepieces exhibiting good color balanceand/or uniformity that may not be achievable through other techniques.For example, in some cases, an eyepiece can be formed from wavelengthselective volume holographic materials (e.g., instead of using thetechniques described herein), but they may have field of viewlimitations stemming from their lower refractive indices (e.g., around1.5).

In general, some materials can have similar refractive indices at aspecific wavelength λ₀, while exhibiting different dispersions (e.g.,refractive index with respect to wavelength). As a result, lightcrossing the interface between these materials will not be affected atthat specific wavelength λ₀, while being affected at other wavelengths.If the interface includes a grating structure, the diffractionefficiency of that grating structure will be close to zero aroundwavelength λ₀, because there would be no phase modulation. However,light would be diffracted at other wavelengths, with a diffractionefficiency that is proportional to the refractive index differencebetween the materials.

As an example, FIG. 7 shows, in cross-section, an example gratingstructure 700 defined at an interface between a first optical element702 a (having a refractive index of n₁) and a second optical element 702b (having a different refractive index n₂). As the diffractionefficiency of that grating structure 700 is close to zero at or aroundwavelength λ₀, light 704 a having a wavelength λ₀ passes through thegrating structure 700 with little or no diffraction. However, light 704a having a different wavelength λ₁ is diffracted as it passes throughthe grating structure 700, with a diffraction efficiency that isproportional to the difference in refractive index between the twomaterials (e.g., the difference between n₁ and n₂).

The spectral response of the grating structure is dictated, at least inpart, by the refractive indices and the dispersion properties of thematerials used to form the grating structure. Accordingly, a particularspectral response of the grating structure can be achieved by selectingcertain materials (e.g., having certain refractive indices and thedispersion properties) to form the grating structure.

Further, spectral response of the grating structure is dictated, atleast in part, by the physical dimensions of the gratings (e.g., theirheight, width, periodicity, duty cycle, etc.). Accordingly, a particularspectral response of the grating structure can be achieved by furtherforming gratings having certain dimensions using the selected materials.

As an example, FIG. 8A shows, in cross-section, a single unit 802 of agrating structure 800. The unit 802 can repeat one or more timesperiodically along a periphery of an optical element (e.g., at aninterface between the optical element and another optical element, oralong an interface between the optical element and air). The unit 802includes a base portion 804 composed of a first material, and protrusion806 composed of the first material and extending from the base portion804. The unit 802 also includes filling portions 808 composed of asecond material different from the first material, and disposed on thebase portion 504 along opposing sides of the protrusion 806. As shown inFIGS. 8B and 8C, the unit 802 repeats periodically along the peripheryof an optical element 810, forming the grating structure 800 (e.g., a“binary” grating in filling portions disposed between each protrusion).In the example shown in FIG. 8B, the optical element 810 receives lightthrough the grating structure 800 (e.g., from a light source 812), andlight 814 incident upon the grating structure 800 is diffracted as itenters the optical element 810. In the example shown in FIG. 8C, theoptical element 810 emits light 816 through the grating structure 800(e.g., towards another optical element, into the air, and/or towards auser's eye), and light incident upon the grating structure 800 isdiffracted as it exits the optical element 810. In some implementations,one or more of the out-coupling optical elements 570, 580, 590, 600, 610(e.g., as shown in FIG. 4) can include a respective grating structure800. In some other instantiations such as those used in waveguidedisplays, the light to be coupled out propagates through the substrateby total internal reflection (TIR) and is extracted from the waveguideby the grating structure.

Referring back to FIG. 8A, the unit 802 has a cross-sectional widthw_(t) (e.g., corresponding to the periodicity of the unit 802), and across-sectional height h_(t) (e.g., corresponding to the maximal heightof the unit 802). Further, the protrusion 506 has a cross-sectionalwidth w₁ and a cross-sectional height h₁ (e.g., the difference in heightbetween the top surface of the protrusion 806 and the top surface of thebase portion 504). The base portion 804 has a cross-sectional widthw_(t), and a cross-sectional height h₃. Each filler portion 808 has across-sectional width w₂, and a cross-sectional height h₂.

Each of the parameters w_(t), w₁. w₂, h_(t), h₁, h₂, h₂, and h₃, can beselected to impart certain optical properties with respect the gratingstructure. Further, the materials of the base portion 804, protrusion806, and filling portions 808 also can be selected (e.g., based at leastin part on their respective refractive indices and the dispersionproperties) to impart certain optical properties with respect thegrating structure. In particular, if the repeating period of the gratingstructures is sufficiently small (e.g., between 250 nm and 400 nm), theextraction efficiency of light propagating by total interface reflection(TIR) within the substrate can be controlled with respect to wavelength.

In some implementations, these parameters can be selected such that thegrating structure exhibits a diffraction efficiency that is uniform ormore uniform over a particular range of incident angles of light andwith respect to particular wavelengths of light (e.g., compared tograting structures designed using techniques different from thosedescribed herein). As an example, the base portion 504 and theprotrusion 506 can be formed from SiC (e.g., through a deposition andetching process), and the filling portions 508 can be formed from TiO₂(e.g., through a deposition process, such as sputtering). In some cases,the refractive index of the SiC portions can be between 2.65 and 2.8,and the refractive index of the TiO2 portions can be between 2.2 and2.6. Further, the grating structure can be formed such that w₁ is equalto or approximately equal to 140 nm (e.g., between 80 nm and 200 nm), w₂is equal to or approximately equal to 34 nm (e.g., between 30 nm and 200nm), h₁ is equal to or approximately equal to 90 nm (e.g., between 40 nmand 150 nm), h₂ is equal to or approximately equal to 80 nm (e.g.,between 40 nm and 150 nm), w_(t) is equal to or approximately equal to208 nm (e.g., between 150 nm and 400 nm. It should be noted that thenumbers given in the example above correspond to the out-coupling ofTIR-guided light within the substrate.

FIG. 9A shows the angular response of a binary grating structure etchedin SiC (e.g., without any filling portions of TiO₂ deposited between theprotrusions of the grating structure) with respect to three differentwavelengths of light (red, green and blue). FIG. 9B shows the angularresponse of a similar binary grating structure etched in SiC—but alsohaving filling portions of TiO₂ deposited between the protrusions of thegrating structure (e.g., as shown in FIGS. 5A-5C)—with respect to thesame three wavelengths of light (red, green and blue). As shown in FIGS.9A and 9B, the addition of the filling portions of TiO₂ increases adiffraction efficiency of the grating structure across a range ofincident angles of light (e.g., from −20° to 20°) with respect to eachof the different wavelengths of light. Accordingly, light incident onthe grating structures described herein exhibit are less likely toexhibit color-dependent or incident angle-dependent diffractioncharacteristics.

As described herein, optical elements having the grating structuresdescribed herein may be particularly suitable for use as eyepieces in awearable display headset. For example, a wearable display headset may beconfigured to display multi-colored images (e.g., RGB images).Accordingly, one or more optical elements of the wearable displayheadset (e.g., the eyepiece and/or any other optical elements) can beformed having the grating structures described herein, such that theyare less likely to exhibit color-dependent or incident angle-dependentdiffraction characteristics. This can facilitate the display ofmulti-color images with improved uniformity (e.g., with respect to lightintensity) over a wide field of view. For instance, referring to FIG. 4,one of the out-coupling optical elements 570, 580, 590, 600, 610 may bediffractive optical features, including the diffractive gratingsdescribed herein.

As an example, FIG. 10A shows an intensity map of light emitted by aneyepiece including a binary grating structure etched in SiC (e.g.,without any filling portions of TiO₂ deposited between the protrusionsof the grating structure, as described with respect to FIG. 6A). FIG.10B shows an intensity map of light emitted by a similar binary gratingstructure etched in SiC—but also having filling portions of TiO₂deposited between the protrusions of the grating structure (e.g., asdescribed with respect to FIGS. 8A-8C and 9B). As shown in FIGS. 10A and10B, the addition of the filling portions of TiO₂ increases theuniformity of projected light. For example, the eyepiece of FIG. 10Aexhibits localized bands of high-intensity light (e.g., a C-shapedartifact of higher-intensity light surrounded by regions oflower-intensity light). In contrast, the eyepiece of FIG. 10B exhibits amore uniform light intensity pattern.

Although example parameters and materials are described here, these aremerely illustrative examples. In practice, one or more parameters maydiffer, depending on the implementation.

Further, different materials can be used other than those describedabove with respect to FIGS. 8A-8C, 9A, 9B, 10A, and 1000B. As anexample, in some implementations, the base portion and the protrusionsof a grating structure can be composed of SiC, LiNbO₃, or a combinationthereof. Further, the filling portions can be composed of TiO₂. In someimplementations, the principles described herein can be applied to othercombination of materials in which the waveguide substrate exhibits ahigher refractive index and a lower dispersion than the coatingmaterial. Examples include diamond/LiNbO₃ and diamond/SrTiO₃ systems.

In some implementations, the refractive index of a material may vary,depending on the manner in which the material is deposited (e.g., on anunderlying substrate). As an example, the refractive index of asputtered layer of titanium dioxide can be varied between 2.25 and 2.65by changing the deposition parameters, such as the temperature and/orthe pressure at which the materials were sputtered onto the underlyingmaterial. Accordingly, the spectral response of a grating structure canbe “tuned” by changing the deposition conditions of one or morematerials used to define the grating structure.

As an example, FIG. 11 shows the refractive index curves of crystallineSiC and TiO₂ deposited according to the atomic deposition (ALD)technique. Each of these materials exhibits a refractive index thatvaries with respect to the incident wavelength of light (e.g., definingparticular refractive index curves). These refractive index curves canbe modified, at least in part, by varying the deposition parameters ofeach of the materials (e.g., the temperature and/or pressure when thematerials are sputtered onto a substrate or other structure).

In the examples shown in FIGS. 8A-8C, each repeating unit of the gratingincludes a protrusion that is rectangular in cross-section (e.g.,forming a binary grating). However, this need not be the case. Forinstance, in some implementations, each unit of the grating can includedifferently shaped protrusions. As an example, as shown in FIGS. 12A and12B, each unit of a grating can include a protrusion that is rectangularin cross-section (e.g., a isosceles triangle as shown in FIG. 12A, aright triangle as shown in FIG. 12B, etc.). In practice, any othergrating configurations also can be used, depending on theimplementation. It should be noted that the technique is also applicableto two-dimensional diffractive lattices such as two-dimensional arraysof rods, squares, or pyramids.

FIG. 13 shows an example process 1300 for constructing a head-mounteddisplay device using the optical elements and grating structuresdescribed herein.

According to the process 1300, a first optical element is provided (step1302). The first optical element includes a grating formed along a firstsurface of the first optical element. The grating includes plurality ofprotrusions including a first material having a first optical dispersionprofile for visible wavelengths of light, and a second materialdeposited between at least some of the plurality of protrusions alongthe first surface of the first optical element. The second material hasa second optical dispersion profile for visible wavelengths of light.Example first optical elements are shown and described with respect toFIGS. 8A-8C.

In some implementations, the second material can be titanium dioxide(TiO₂). In some implementation, the first material can be siliconcarbide (SiC) or lithium niobate (LiNbO₃).

In some implementations, the grating can be formed by etching aplurality of channels onto the first optical element along the firstsurface. Each channel can have a first depth. Further, the secondmaterial can be deposited between at least some of the plurality ofprotrusions along the first surface. An example of this configuration isshown, for example, in FIGS. 8A-8C.

In some implementations, each channel can have a substantiallyrectangular cross-section. In some implementations, each channel canhave a substantially equal width (e.g., approximately 68 nm).

In some implementations, depositing the second material can includedepositing the second material into at least some of the channels.

In some implementations, depositing the second material can includesputtering the second material into at least some of the channels. Thesecond material can be sputtered at different temperatures and/orpressures to varying the optical properties (e.g., the refractive index)of the material.

In some implementations, the second material can be deposited such thatit extends a first height within the channel.

In some implementations, the first depth can be greater than the firstheight. As an example, the first depth can be approximately 90 nm, andthe first height can be approximately 80 nm.

In some implementations, the grating can be formed according to a periodalong a length of the first surface. As an example, the period cancorrespond to a length of approximately 208 nm.

Further, the first optical element is positioned in opticalcommunication with a second optical element in the head-mounted displaydevice (step 1304). Example configurations of a first optical elementand a second optical element in a head-mounted display device are shownand described with respect to FIGS. 1 and 4-6.

Some implementations of subject matter and operations described in thisspecification can be implemented in digital electronic circuitry, or incomputer software, firmware, or hardware, including the structuresdisclosed in this specification and their structural equivalents, or incombinations of one or more of them. For example, in someimplementations, the local processing and data module 140, the remoteprocessing module 150, and/or the remote data repository 160 can beimplemented using digital electronic circuitry, or in computer software,firmware, or hardware, or in combinations of one or more of them. Inanother example, the process 1300 shown in FIG. 1300 can be implemented,at least in part, using digital electronic circuitry, or in computersoftware, firmware, or hardware, or in combinations of one or more ofthem (e.g., as a part of an automated or computer-assisted manufacturingprocess).

Some implementations described in this specification can be implementedas one or more groups or modules of digital electronic circuitry,computer software, firmware, or hardware, or in combinations of one ormore of them. Although different modules can be used, each module neednot be distinct, and multiple modules can be implemented on the samedigital electronic circuitry, computer software, firmware, or hardware,or combination thereof.

Some implementations described in this specification can be implementedas one or more computer programs, i.e., one or more modules of computerprogram instructions, encoded on computer storage medium for executionby, or to control the operation of, data processing apparatus. Acomputer storage medium can be, or can be included in, acomputer-readable storage device, a computer-readable storage substrate,a random or serial access memory array or device, or a combination ofone or more of them. Moreover, while a computer storage medium is not apropagated signal, a computer storage medium can be a source ordestination of computer program instructions encoded in an artificiallygenerated propagated signal. The computer storage medium can also be, orbe included in, one or more separate physical components or media (e.g.,multiple CDs, disks, or other storage devices).

The term “data processing apparatus” encompasses all kinds of apparatus,devices, and machines for processing data, including by way of example aprogrammable processor, a computer, a system on a chip, or multipleones, or combinations, of the foregoing. The apparatus can includespecial purpose logic circuitry, e.g., an FPGA (field programmable gatearray) or an ASIC (application specific integrated circuit). Theapparatus can also include, in addition to hardware, code that createsan execution environment for the computer program in question, e.g.,code that constitutes processor firmware, a protocol stack, a databasemanagement system, an operating system, a cross-platform runtimeenvironment, a virtual machine, or a combination of one or more of them.The apparatus and execution environment can realize various differentcomputing model infrastructures, such as web services, distributedcomputing and grid computing infrastructures.

A computer program (also known as a program, software, softwareapplication, script, or code) can be written in any form of programminglanguage, including compiled or interpreted languages, declarative orprocedural languages. A computer program may, but need not, correspondto a file in a file system. A program can be stored in a portion of afile that holds other programs or data (e.g., one or more scripts storedin a markup language document), in a single file dedicated to theprogram in question, or in multiple coordinated files (e.g., files thatstore one or more modules, sub programs, or portions of code). Acomputer program can be deployed to be executed on one computer or onmultiple computers that are located at one site or distributed acrossmultiple sites and interconnected by a communication network.

Some of the processes and logic flows described in this specificationcan be performed by one or more programmable processors executing one ormore computer programs to perform actions by operating on input data andgenerating output. The processes and logic flows can also be performedby, and apparatus can also be implemented as, special purpose logiccircuitry, e.g., an FPGA (field programmable gate array) or an ASIC(application specific integrated circuit).

Processors suitable for the execution of a computer program include, byway of example, both general and special purpose microprocessors, andprocessors of any kind of digital computer. Generally, a processor willreceive instructions and data from a read only memory or a random accessmemory or both. A computer includes a processor for performing actionsin accordance with instructions and one or more memory devices forstoring instructions and data. A computer may also include, or beoperatively coupled to receive data from or transfer data to, or both,one or more mass storage devices for storing data, e.g., magnetic,magneto optical disks, or optical disks. However, a computer need nothave such devices. Devices suitable for storing computer programinstructions and data include all forms of non-volatile memory, mediaand memory devices, including by way of example semiconductor memorydevices (e.g., EPROM, EEPROM, flash memory devices, and others),magnetic disks (e.g., internal hard disks, removable disks, and others),magneto optical disks, and CD ROM and DVD-ROM disks. The processor andthe memory can be supplemented by, or incorporated in, special purposelogic circuitry.

To provide for interaction with a user, operations can be implemented ona computer having a display device (e.g., a monitor, or another type ofdisplay device) for displaying information to the user and a keyboardand a pointing device (e.g., a mouse, a trackball, a tablet, a touchsensitive screen, or another type of pointing device) by which the usercan provide input to the computer. Other kinds of devices can be used toprovide for interaction with a user as well; for example, feedbackprovided to the user can be any form of sensory feedback, e.g., visualfeedback, auditory feedback, or tactile feedback; and input from theuser can be received in any form, including acoustic, speech, or tactileinput. In addition, a computer can interact with a user by sendingdocuments to and receiving documents from a device that is used by theuser; for example, by sending web pages to a web browser on a user'sclient device in response to requests received from the web browser.

A computer system may include a single computing device, or multiplecomputers that operate in proximity or generally remote from each otherand typically interact through a communication network. Examples ofcommunication networks include a local area network (“LAN”) and a widearea network (“WAN”), an inter-network (e.g., the Internet), a networkcomprising a satellite link, and peer-to-peer networks (e.g., ad hocpeer-to-peer networks). A relationship of client and server may arise byvirtue of computer programs running on the respective computers andhaving a client-server relationship to each other.

FIG. 14 shows an example computer system 1400 that includes a processor1410, a memory 1420, a storage device 1430 and an input/output device1440. Each of the components 1410, 1420, 1430 and 1440 can beinterconnected, for example, by a system bus 1450. The processor 1410 iscapable of processing instructions for execution within the system 1400.In some implementations, the processor 1410 is a single-threadedprocessor, a multi-threaded processor, or another type of processor. Theprocessor 1410 is capable of processing instructions stored in thememory 1420 or on the storage device 1430. The memory 1420 and thestorage device 1430 can store information within the system 1400.

The input/output device 1440 provides input/output operations for thesystem 1400. In some implementations, the input/output device 1440 caninclude one or more of a network interface device, e.g., an Ethernetcard, a serial communication device, e.g., an RS-232 port, and/or awireless interface device, e.g., an 802.11 card, a 3G wireless modem, a4G wireless modem, etc. In some implementations, the input/output devicecan include driver devices configured to receive input data and sendoutput data to other input/output devices, e.g., keyboard, printer anddisplay devices 1460. In some implementations, mobile computing devices,mobile communication devices, and other devices can be used.

While this specification contains many details, these should not beconstrued as limitations on the scope of what may be claimed, but ratheras descriptions of features specific to particular examples. Certainfeatures that are described in this specification in the context ofseparate implementations can also be combined. Conversely, variousfeatures that are described in the context of a single implementationcan also be implemented in multiple embodiments separately or in anysuitable subcombination.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made without departingfrom the spirit and scope of the invention. Accordingly, otherimplementations are within the scope of the following claims.

1. (canceled)
 2. A system comprising: a plurality of optical elements inoptical communication, the plurality of optical elements beingconfigured, during operation, to project an image in a field of view ofa user, wherein a first optical element of the plurality of opticalelements is configured to receive light from a second optical element ofthe plurality of optical elements; wherein the first optical elementdefines a grating at along a periphery of the first optical element, thegrating comprising: a plurality of protrusions extending from a baseportion of the first optical element, the protrusions comprising a firstmaterial having a first refractive index for a visible wavelength oflight, and each of the protrusions comprising a first surface oppositethe base portion of the first optical element; and a second materialdisposed between at least some of the plurality of protrusions along thebase portion of the first optical element, the second material having asecond refractive index for the visible wavelength of light, whereinthere is an absence of the second material on the first surfaces of theprotrusions.
 3. The system of claim 2, wherein the second materialcomprises titanium dioxide (TiO₂).
 4. The system of claim 3, wherein thefirst material comprises silicon carbide (SiC).
 5. The system of claim3, wherein the first material comprises lithium niobate (LiNbO₃).
 6. Thesystem of claim 2, wherein the base portion of the first optical elementcomprises the first material.
 7. The system of claim 2, wherein the baseportion of the first optical element comprises the same material as theplurality of protrusions.
 8. The system of claim 2, wherein the baseportion of the first optical element is integrally formed with theplurality of protrusions.
 9. The system of claim 2, wherein eachprotrusion has a substantially rectangular cross-section.
 10. The systemof claim 2, wherein each protrusion extends a first height above asurface of the base portion of the first optical element, and whereinthe second material extends a second height above the surface of thebase portion of the first optical element, the second height beingdifferent from the first height.
 11. The system of claim 10, wherein thefirst height is greater than the second height.
 12. The system of claim11, wherein the first height is approximately 90 nm.
 13. The system ofclaim 12, wherein the second height is approximately 80 nm.
 14. Thesystem of claim 2, wherein the grating repeats according to a periodalong a length of the base portion of the first optical element.
 15. Thesystem of claim 14, wherein the period corresponds to a length ofapproximately 208 nm.
 16. The system of claim 2, wherein each protrusionhas a substantially equal width.
 17. The system of claim 2, wherein eachprotrusion has a width of approximately 140 nm.
 18. A method comprising:providing a first optical element comprising a grating formed along afirst surface of the first optical element, the grating comprising: aplurality of protrusions comprising a first material having a firstrefractive index for a visible wavelength of light, and each of theprotrusions comprising a first surface opposite the base portion of thefirst optical element, and a second material deposited between at leastsome of the plurality of protrusions along the first surface of thefirst optical element, the second material having a second refractiveindex for the visible wavelength of light, wherein there is an absenceof the second material on the first surfaces of the protrusions; andpositioning the first optical element in optical communication with asecond optical element.
 19. The method of claim 18, wherein the secondmaterial comprises titanium dioxide (TiO₂).
 20. The method of claim 19,wherein the first material comprises silicon carbide (SiC).
 21. Themethod of claim 19, wherein the first material comprises lithium niobate(LiNbO₃).